Understanding Sulfur Content in Alkylate from Sulfuric Acid-Catalyzed C₃/C₄ Alkylations
2019
Minnick, David L. | Kore, Rajkumar R. | Lyon, Christopher J. | Subramaniam, Bala | Shiflett, Mark B. | Scurto, Aaron M.
Alkylation of short-chain olefins with isobutane catalyzed by sulfuric acid is a common process for reformulated fuel. Here, pilot-plant and commercial C₃ and C₄ alkylates were examined for sulfur content, acid content, and emulsion formation. Even though the thermodynamic solubility of sulfuric acid in alkylate is negligible at process conditions, the C₄ alkylate samples contained ∼20 ppm sulfur mostly from very dilute emulsions with ∼3 μm droplets of sulfuric acid and alkyl sulfates that were stable even after 6 months. The sulfur content and droplet size increased for propylene alkylation. However, no detectable emulsion or sulfur content could be generated synthetically by intense mixing with either 2,2,4-trimethylpentane (a model alkylate) or a treated pilot-plant alkylate with concentrated or spent sulfuric acid over the course of several hours. Thus, the alkylate sulfur content is most likely created during the acid-catalyzed chemical reaction steps and not from high-shear mixing.
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